1. Field of the Invention
This invention relates to a linear image sensor made of a bipolar integrated circuit technique and more particularly to an elongate integrated image sensor used for an original read scanner or the like in order to read image-formation on the sensor, isometric to the original.
2. Description of the Prior Art
The image sensor using the integrated circuit technique and formed on silicon crystals includes a charge-coupled device (CCD) image sensor and a metal oxide semiconductor (MOS) image sensor. Generally, the image sensor comprises a plurality of photo-detecting elements and scanning circuits, the CCD image sensor using a photodiode for the detecting element and a charge coupled device for the scanning circuit, and the MOS image sensor using a photodiode for the detecting element and a shift register of MOS transistor for the scanning circuit. These image sensors each comprise the photodetector unit and scanning circuit unit on the basis of MOS integrated circuit technique and have high resolution, but are defective in the following matters: The CCD image sensor, which uses, as the signal medium, charge carriers essentially in thermal nonequilibrium condition, becomes larger in dark signal at a high temperature. Also, in a case of reading contracted image-formation, the potential well size of CCD is small (7 to 14 .mu.m) to enable high speed scanning, but in a case of reading the isometric image, the necessary potential well size is larger (62 to 125 .mu.m) to result in low speed scanning, in turn read-out at low speed. On the other hand, the MOS image sensor also of high resolution is slow at the scanning speed due to the limit in the maximum clock frequency of MOS shift register and also its video signal is weak and the scanning digital signal interferes intensively with the video signal line, resulting in a poor signal-to-noise ratio.
Recently, for miniaturization, high resolution and easy adjustment of the original read scanner, a contact type image sensor to read images isometric to the original has largely been developed, which has relatively larger element pitch (for example, pitch: 62.5 .mu.m at a resolution of 16 dots/mm and pitch: 125 .mu.m at that of 8 dots/mm) and needs a length corresponding to a width of the original. To meet the above requirement, there are an amorphous silicon image sensor whose photo-detecting units are integrally elongate and a large number of driving LSI chips are mounted therearound, and a CCD multichip image sensor which mounts a plurality of CCD image sensors zigzag on a substrate. The amorphous silicon image sensor, whose detecting units are separate from driving units, has many connecting wires with each other and is problematical in reliability, the CCD multichip image sensor being defective in that rearrangement of lines after read-out is required due to the zigzag arrangement, the transfer speed lowers due to its large potential well size, and a capacity load of driving circuit increases.